A mass sensor is provided for measuring a particle mass within an aerosol. The duration of a sensing cycle is set such that a pre-set change in mass resulting from particles deposited is caused. In the absence of cleaning, the lifetime of the sensor is dependent on the total mass deposited. As a result, the lifetime is made essentially constant by this approach, because each sensing operation is made to give rise to a constant amount of deposited particle mass. This means the lifetime can be predicted more accurately.